Optical telecommunication networks have been widely built out in recent years and are gaining in popularity. Optical fibers are capable of carrying a high volume of traffic at a reasonable cost. Although previously the “last mile” between an optical communication network and the end user was still spanned with copper wire, now fiber optic cables are often run directly up to houses, apartment complexes, and business locations. One arrangement for spanning this portion of the communication link is referred to as a PON.
Generally speaking, a PON includes an optical line termination (OLT), typically located in a central office, which communicates via a fiber-optic cable system with one or more optical network units (ONUs), with each ONU being located on or near a customer premises. An optical network termination (ONT) is an ONU that typically serves a single user and may, for example, be located at the user's residence. A multi-dwelling unit (MDU) is an ONU that serves a multi-dwelling unit such as an apartment complex or small business. Each ONU is capable of segregating the downstream signals from the OLT and directing them to the proper user, and of transmitting upstream signals back to the OLT. In addition to forming a part of one or more PONS, the OLT is also connected to the larger telecommunication system through which the various services such as telephony, Internet access, and broadcast media are accessible so that they can be made available to the users associated with the OLT.
Standards have been promulgated for PON operations. For example, many current implementations are configured in accordance with a family of specifications including ITU-T G.984 and related standards. Such systems currently provide for transmission speeds of up to (approximately) 2.5 Gbps in the downstream direction and 1.24 Gbps upstream. The directional difference in transmission speeds is due in part to practical consideration of the cost of facilitating higher upstream transmission speeds, coupled with the fact that, as a general rule, more content needs to be transmitted downstream than back to the OLT.
With increased utilization of optical network services, however, a need exists to increase existing aggregated date rate, at least in the downstream direction and eventually in both directions. One solution, of course, is simply to replace or upgrade all PONs and related equipment with the components necessary to accommodate the higher aggregated date rate. This, however, may be too expensive or difficult, especially in the near term.
For example, the best existing solution in supporting a higher aggregated date rate is a tunable wavelength division multiplexing (TWDM) PON, e.g., ITU-T NGPON2. Unfortunately, this solution requires a WDM filter at the OLT and/or array waveguide gratings (AWGs) in the optical distribution network (ODN) and tunable lasers in the ONT. Apart from the high component cost of each AWG, the WDM filter and the tunable laser, replacing conventional optical power splitters with AWGs in the ODN is costly.